The preparation of methanol is based on following three equilibrium reactions:CO+2H2<=>CH3OH  (1)CO2+3H2<=>CH3OH+H2O  (2)CO+H2O<=>CO2+H2  (3)
Due to the equilibrium only a fraction of the synthesis gas is converted to methanol and the remaining part of the synthesis gas has to be recycled. In situ separation of methanol from the synthesis gas is disclosed in U.S. Pat. No. 4,731,387. In a gas solid trickle flow reactor the methanol is removed by an absorption material and thereby the equilibrium condition improves. After having passed the reactor the methanol is desorbed from the absorption material and the absorption material is recycled to the inlet of the reactor. The drawbacks of such system lie in the complexity of the system, which results in operational difficulties and a higher investment cost.
Another way of overcoming the equilibrium limitations is disclosed in U.S. Pat. No. 5,262,443, where the catalytic reactor is operated at a temperature and pressure where a part of the produced methanol condensates in the catalytic bed. By applying this invention, it is possible to reduce of eliminate the expensive synthesis gas recycle. There are, however, two drawbacks by operating in this way.
In order to operate below the gas dew point, the catalyst temperature has to be reduced below the optimal temperature level for the catalytic reaction. The lower temperature results in a lower activity, which increases the necessary catalyst volume and cost of the reactor.
The second problem involves the condensation of methanol in the porous catalyst. The synthesis gas has to diffuse inside the catalyst through the pore system to initiate the catalytic reaction. If the pores are filled with methanol, the diffusion rate and catalytic activity are severely reduced.
These two problems reduce the catalyst activity several times compared to the activity obtained in the conventional methanol synthesis process. As a consequence of the reduced activity the condensing reactor has to be increased in size resulting in reactors being more expensive than conventional reactors with recycle of synthesis gas.